Interpretive Summary: Intramammary infection by Staphylococcus aureus continues to be a major concern among dairymen. Despite adoption of management practices such as pre- and postmilking teat dipping, dry cow therapy and culling of infected animals, dairymen continue to have problems eliminating intramammary infections by S. aureus from their herds. While the general perception among dairymen is that mastitis in fresh heifers is very low, it has been reported that intramammary infection of fresh heifers provides a source for the introduction of S. aureus into herds. To determine if fresh heifers were serving as a source of S. aureus, the present study was conducted to determine the incidence of intramammary infection at the time of parturition for first calf heifers and multiparous cows. Aseptic quarter foremilk samples (10 ml) were obtained within 3 days of calving from 116 heifers (463 quarters) and 101 multiparous cows (404 quarters) from October 2003 to the present. Intramammary infections were observed in 183 quarters (40%) of 87 heifers, and in 117 quarters (29%) of 59 multiparous cows. For all cows, the data indicate a prevelance of 7% and 2%, respectively, in the quarters of fresh heifers and multiparous cows infected with S. aureus. Fresh heifers are a source for the introduction of S. aureus intramammary infections into a dairy herd.

Technical Abstract:
Aseptic quarter foremilk samples (10 ml) were obtained within 3 days of calving from 116 heifers (463 quarters) and 101 multiparous cows (404 quarters). Intramammary infections were observed in 183 quarters (40%) of 87 heifers, and in 117 quarters (29%) of 59 multiparous cows. Of the 183 infected quarters for heifers, 117 (64%) were infected with coagulase negative staphylococci, 22 (12%) with Gram-negative bacteria, 31 (17%) with S. aureus, 4 (2%) with Streptococcus spp, and 9 (5%) with Cornynebacterium bovis. For multiparous cows, 77 (66%) were infected with coagulase negative staphylococci, 16 (14%) with Gram-negative bacteria, 8 (7%) with S. aureus, 6 quarters (5%) with Streptococcus spp, and 5 (4%) with C. bovis. For all cows, the data indicate a prevelance of 7% and 2%, respectively, in the quarters of fresh heifers and multiparous cows infected with S. aureus. Coagulase negative staphylococci accounted for the majority of bacteria isolated from quarters of both fresh heifers and multiparous cows. A greater prevalence of intramammary infections by S. aureus was observed in fresh heifers than in multiparous cows. Fresh heifers are a source for the introduction of S. aureus intramammary infections into the BARC dairy herd.